Start with the stealing of Albert Einstein’s brain – after he died, of course. That would be the dastardly deed by Dr. Thomas Harvey, the pathologist who performed the 1955 autopsy on the deceased scientist. His purpose was benign: to test whether Einstein’s brain was significantly different from an ordinary person’s, in order to account for his genius.

But “Incognito,” the mystifying, strangely moving play by Nick Payne, isn’t really about that act, although it brings in Einstein’s children and probes how they feel about having their dad’s brain spirited away. Rather, the play asks, What is the “self”? Is it a construct, a series of neurological synapses that come together to try to make sense of events? Or is it filled with conscious choices under our control? Further, what attracts one person to another? How do we deal with our relationship to the world?

Heady stuff yet, as he demonstrated in the haunting, even more moving “Constellations,” Payne couches his explorations not in abstractions but real human beings. Countering Einstein’s brain is that of real-life Henry Maison whose epileptic seizures and inability to create new memories are dealt with by well-meaning therapists. If only they could get him to play the piano again. If only they could convince him about his past. If only . . .

As embodied by the terrific British actor, Charlie Cox, Henry is the flip side of Einstein. Here’s the brain not of a genius, but a troubled man who doesn’t function at all well. Various people – therapists, friends, partners -- flit in and out of his life. We don’t know why he is as he is, just as we don’t really know what it was about Einstein’s brain power and, more personally, we don’t really know what makes us whatever we are.

It may come down to what Einstein’s daughter finally says: that it wasn’t that her father’s brain was unusual, just that he “worked himself to death,” neglecting the human care and connections that his family sought. The brain can be measured in centimeters and folds, impulses and electricity, but the less knowable, more abstract mind cannot.

Told in three parts – “Encoding,” “Storing,” “Retrieving” – with interludes of seemingly nonsensical body and arm movements (movement direction by Peter Pucci), the vignettes are linked through Martha Murphy, a therapist who, in dealing with minds, has relationship problems of her own. What she learns about herself and, by extension, us and the stories we tell ourselves, is the play’s crux. We are as capable of murder as we are of compassion.

Aphorisms guide us: “A damaged brain can still make sense of the world”; “If you can’t remember who you are, you’re not anyone”; “Chance favors the prepared mind”; “Imagination is more important that knowledge.”

Four actors play 20 roles, switching characters and accents with a breath, clarifying what could be obscure. Geneva Carr, Heather Lind and Morgan Spector create distinct people. Cox is just wonderful and Doug Hughes’ direction is so precise that we never lose these multiple threads. Add Scott Pask’s austere set, Ben Stanton’s clever lighting, Catherine Zuber’s muted costumes and David Van Tieghem’s evocative music and sound for an evening that both tests and satisfies.